This invention relates to a parking brake and includes structure to compensate for dimensional changes in a disc brake caused by thermal expansion and contraction to sustain an initial clamping force developed to effect a parking brake application a substantially constant level for the duration of a parking brake application.
It was common for vehicles to have with drum brakes on the front and rear wheels however after a number of years the drum brakes on the front wheels were replaced with disc brakes. Later the drum brakes associated with the rear wheel were replaced with disc brakes such that now it is not unusual for disc brake to be located on all four wheels of a vehicle. When disc brakes are located on the rear wheel of a vehicle it was necessary to revise the parking brake structure In order to provide a total brake system. The importance of a parking brake is of great importance in a vehicle having a manual transmission as the parking brake is utilized to hold a vehicle in a stationary location and in particular if the manual gear is in neutral. In the manufacture of a vehicle a customer is usually is a choice for the structure for a parking brake as illustrated by the drum-in-hat brake disclosed in U.S. Pat. No. 5,180,037, the integral piston brake disclosed in U.S. Pat. No. 5,038,895 or a separate brake as disclosed in U.S. Pat. No. 5,921,354. In each of the parking brake systems, a input force from an operator is applied through a lever to mechanically move friction members into engagement with a drum or a rotor associated with a wheel to effect a parking brake application. While these systems function in an adequate manner for most brake applications should a number of service brake applications be made in a relative short period of time, the temperature created during a brake application may cause significant thermal expansion in the disc brake. Unfortunately should such thermal expansion occur and a mechanical force is applied to effect a parking braking application as the disc brake cools the initial clamping force developed to initiate the parking brake application is reduced as both the rotor and caliper undergo a thermal contraction and as a result the clamping force changes such that the friction engagement of the pads with the rotor may not be adequate to hold a vehicle on an incline surface. A solution to this type situation, defined as creep, is addressed in U.S. Pat. No. 5,921,354 where a self-energizing structure is introduced to modify the force on a separate parking brake on movement of rotor after a parking brake is applied. While this solution may achieve a desired result, the added cost and additional brake components have not yet been accepted by the industry
A primary object of the present invention is to provide a disc brake having an integral parking brake with means to compensate for the effect of thermal expansion and contraction of a disc brake such that a clamping force initiated to effect a parking brake application remains static from inception until termination in urging friction members into engagement with a rotor.
In more particular detail, the disc brake according to the present invention has a support member secured to a vehicle. The support member has first and second guides that align a caliper over a rotor associated with a wheel of the vehicle. The caliper has an actuation section with a bore therein for retaining a piston to define an actuation chamber. The actuation section is connected to an arm by a bridge that extends over the rotor. A first friction member associated with the piston is located on one side of the rotor and a second friction member connected to the arm is located on the other side of the rotor such that the first and second friction members are aligned with each other on opposite sides of the rotor. In response to an operator input, pressurized fluid from a source is presented the actuation chamber to effect a service brake application. The pressurized fluid acts on the piston and actuation section to develop a dynamic clamping force that urges the first and second friction members into engagement with the rotor to effect a corresponding first or service brake application. The length of time and force applied to develop a service brake application varies and is under the constant control of an operator such that when the operator has determined that sufficient braking has been achieved to produce a desired reduction in speed or actually brought a vehicle to a stop, the operator may remove the input force and the dynamic clamping force acting on the piston and actuator section is terminated. The parking brake structure in this disc brake assembly is characterized by a ball-in-ramp actuator and a screw adjustment mechanism through which a static clamping brake force is applied to effect a parking brake application. The ball-in-ramp actuator has a first member that is axially retained in the actuation chamber and is connected to a lever mechanically linked to receive an input force from an operator and a second member that is radially retained in the actuation chamber while the adjustment screw mechanism has a first end that is connected to the second member of the ball-in-ramp actuator and a second end that engages the piston. The first member of the ball-in-ramp actuator responds to an input force applied to the lever by rotating and moving ball in a ramp to axially move the second member and transmit an actuation force through the adjustment screw mechanism into the piston to produce a second or static clamping force. The static clamping force acts on and moves the piston and actuation section such that the first and second friction members are urged into engagement with the rotor to effect a second or parking brake application. The caliper, support member and rotor are susceptible to thermal expansion and thermal contraction that may cause dimensional changes. The adjustment screw mechanism is characterized by spring means that compensates for dimensional changes in the caliper, support member and rotor by sustaining a substantially same static actuation force on piston such that the clamping force remains static for the duration of a parking brake application.
An advantage of the disc brake provided by the present invention resides in a parking brake having a static clamping force to maintain a substantially same level of engagement force between friction pads and a rotor during a parking brake application.
An object of this invention is to provide a disc brake assembly with spring means to compensate for dimensional changes that may occur as a result of thermal contraction in a rotor and caliper such that a clamping force remains static for the duration of a parking brake application.